High manganese duplex stainless steel having superior hot workabilities and method for manufacturing thereof

ABSTRACT

A high manganese duplex stainless steel with excellent hot workability, comprising (in weight %): less than 0.1% of C; 0.05-2.2% of Si; 2.1-7.8% of Mn; 20-29% of Cr; 3.0-9.5% of Ni; 0.08-0.5% of N; less than 5.0% of Mo and 1.2-8% of W, alone or composite; the balance Fe and inevitable impurities; and a method for manufacturing the duplex stainless steel, comprising the steps of: solution heating the duplex stainless steel composition at a temperature of 1,050 to 1,250° C., hot working at a starting temperature of 1,130 to 1,280° C. and then ending at a temperature greater than 1,000° C., and then cooling within the temperature range from 1,000 to 700° C. at a cooling rate of more than 3° C./min. The duplex stainless steel exhibits a reduction in area of more than 50% at 1,050° C., and possesses a yield strength of more than 400 MPa, and a corrosion rate of less than 0.36 mm/year, after solution heating.

TECHNICAL FIELD

[0001] The present invention relates to a duplex stainless steel usefulfor structural parts requiring strength and corrosion resistance, andmore particularly, to a high manganese duplex stainless steel havingexcellent hot workability and a method for manufacturing the same.

BACKGROUND ART

[0002] Hitherto, duplex stainless steels have widely been used as basicmaterials in industrial equipments and structural parts requiringoxidation resistance and corrosion resistance. In particular, because2205 type duplex stainless steels have higher corrosion resistance thanaustenite type stainless steels and are high in strength, they have beenwidely used in pipelines for chemical equipments, structural parts fordechlorination and desulfurization in power plants and the petrochemicalindustry, internal screw conveyors or bleaching reservoirs in the papermanufacturing industry, marine related equipments and the like.Recently, demands for duplex stainless steels have been increasing,because establishments of dechlorination and desulfurization systems arerequired in electric power stations or petrochemical equipmentsaccording to air pollution prevention policy. In addition to the above,they have been used as essential materials for air purificationequipments in industrial waste incinerators.

[0003] Duplex stainless steels consist of ferrite phase and austenitephase, the ferrite phase improving strength and the austenite phaseimproving corrosion resistance. Duplex stainless steels are known thatpitting corrosion resistance and crevice corrosion resistance increase,resulting from inclusion of Cr, Mo, W, and N in a basic Fe (R. N. Gunn,“Duplex Stainless Steels”, Woodhead Publishing Ltd., (1997)). Afterduplex stainless steels are subjected to casting or solution heattreatment, if they are not cooled at an appropriate rate, precipitatescontaining large amounts of Mo or W, including mainly sigma phase, areformed within the temperature range of 700 to 950° C. Furthermore, 60′-phase forming zone is within the temperature range of 300 to 350° C.Precipitates formed at high or medium temperature improve hardness ofduplex stainless steels. However, there are problems in thatroom-temperature ductility and impact toughness drastically deteriorateand corrosion resistance drops.

[0004] Typically, commercial Mo-containing duplex stainless steels havea basic chemical composition of Fe-(21-23 wt %)Cr-(4.5-6.5 wt%)Ni-(2.5-3.5 wt %)Mo-(0.08-0.20 wt %)N, and further contain less than2.0% of Mn and less than 0.03% of C (UNS31803 or SAF 2205). There existSAF 2507 type duplex stainless steels with superior corrosionresistance, resulting from increasing contents of Cr and Mo in the 2205type duplex stainless steels. They have a basic chemical composition ofFe-(24-26 wt %)Cr-(6-8 wt %)Ni-(3-5 wt %)Mo-(0.24-0.32 wt %)N andfurther contain less than 1.2% of Mn and less than 0.03% of C.

[0005] U.S. Pat. No. 4,657,606 discloses duplex stainless steels have abasic chemical composition of Fe-(23-0.27 wt %)Cr-(4-7 wt %)Ni-(2-4 wt%)Mo-(less than 0.08 wt %)C. It has been reported that if the content ofCu is limited to 1.1-3.0% and the content of Mn increases up to 5-7%,after solution heating and then cooling, rapid formation of sigma- orα′-phase is inhibited, thereby room-temperature ductility beingenhanced. However, these sorts of steels are poor in hot workability.

[0006] Meanwhile, many techniques have attempted to increase content ofMn, considering the fact that Mn improves room-temperature ductility andincreases solid solubility of nitrogen by replacing expensive Ni. U.S.Pat. No. 4,272,305 discloses that the content of N is defined as high as0.35-0.6% and the content of Mn is increased up to 4-6%, resulting inincreasing solid solubility of nitrogen in a duplex stainless steel ofFe-(22-28 wt %)Cr-(3.5-5.5 wt %)Ni-(1-3 wt %)Mo-(less than 0.1 wt %)C.However, this sort of steel has a disadvantage in that, due to highcontent of nitrogen, castability and hot workability deteriorate. And,U.S. Pat. No. 4,828,630 discloses that the content of Mn is increased upto 4.25-5.5%, thereby replacing expensive Ni and increasing solidsolubility of nitrogen in a duplex stainless steel of Fe-(17-21.5 wt%)Cr-(1-4 wt %)Ni-(less than 2 wt %)Mo-(less than 0.07 wt %)C. However,this sort of steel has a problem in that the lower limit of Ni is low,capable of adversely influencing corrosion resistance. Japanese PatentLaid-Open Publication No. 9-31604 discloses that the content of Si ismaintained to be high (2.5-4.0%), and in order to increase solidsolubility of nitrogen, the content of Mn is increased to be 3-7% in aMo—W containing duplex stainless steel. However, this sort of steel hasa problem in that, due to excess of Si, impact toughness deteriorates.Accordingly, it is difficult for this sort of steel to becommercialized.

[0007] Meanwhile, there have been some attempts to add Mn to a Fe—Cr—Nitype austenite stainless steel known as 304 or 316 type stainless steel,in order to replace expensive Ni. However, as the added amount of Mnincreases, hot workability deteriorates and thus satisfactory resultsare not obtained. This fact was reported in T. M. Bogdanova et al.,Structure and Properties of Nonmagnetic Steels, Moscow, USSR, pp.185-190, (1982). And, it has been reported that as a result of includingMn and S in 316L, 309S, and 310S type stainless steels, if the contentof Mn is higher, re-precipitation or segregation of S is easier, therebydeteriorating hot workability (S. C. Lee et al., 40th Mechanical Workingand Steel Processing Conf., Pittsburgh, Pa., USA, pp.25-28, (1998)).

[0008] Accordingly, in most commercial duplex stainless steels, toensure hot workability, the content of Mn is limited to less than 2%.For example, U.S. Pat. No. 4,664,725 discloses that although hotworkability is improved in a Ca/S ratio of greater than 1.5, the upperlimit of Mn must be defined, since as addition of Mn increases, hotworkability and corrosion resistance deteriorate.

[0009] As seen from the above, it is commonly regarded that as thecontent of Mn increases, hot workability deteriorates in duplexstainless steels. U.S. Pat. No. 4,101,347 proposes that the content ofMn should be limited to less than 2%, so as to prevent formation ofsigma phase in the duplex stainless steel. This is supported by the factthat the content of Mn has been limited to less than 2% both inconventional Mo- or Mo—W containing duplex stainless steels.

[0010] Meanwhile, it is known that a Mo—W containing duplex stainlesssteel has an enhanced corrosion resistance. Therefore, recently, studieshave been made on duplex stainless steels in which both Mo and W areadded. For example, in a duplex stainless steel which was proposed by B.W. Oh et al., a part of Mo is replaced with W in a steel which containsless than 2.0% of Mn and 20-27% of Cr (Innovation of Stainless Steel,Florence, Italy, p.359, (1993) or Korean Patent Application No.94-3757). It is reported that a duplex stainless steel containing 1-4%of W and less than 1% of Mo has an improved corrosion resistancecompared with that containing 2.78% of Mo. However, the above steel hasan excessively low W and Mo content, and thus, the corrosion resistanceis relatively decreased.

[0011] For another example, U.S. Pat. No. 5,298,093, filed by SumitomoMetal Industries, Ltd., proposes that 2-4% of Mo and 1.5-5% of W arecontained in a duplex stainless steel in which less than 1.5% of Mn and23-27% of Cr are added. This steel is known to have high strength andexcellent corrosion resistance. However, this steel is liable to crackduring a hot rolling, and because it is a high-alloyed steel, the phasestability tends to be lowered, forming sigma phase, therebydeteriorating corrosion resistance and impact toughness. The W—Mocontaining duplex stainless steel also has a problem in that hotworkability is poor at the time of manufacturing finished product forms,including plate, wire, bar and pipe by hot working, similar to the aboveMo-containing duplex stainless steel. As a result, a defectiveproportion of the products increases.

[0012] Similarly, U.S. Pat. No. 5,733,387 proposes that 1-2% of Mo and2-5% of W are contained in a W—Mo containing duplex stainless steel inwhich less than 2.0% of Mn and 22-27% of Cr are added. However, thisstainless steel still has little enhancement in hot workability,relative to the duplex stainless steel of U.S. Pat. No. 5,298,093.

[0013] In addition, U.S. Pat. No. 6,048,413 proposes a duplex stainlesssteel, in which less than 3.5% of Mn, 5.1-8% of Mo and less than 3% of Ware contained. This steel is a high-alloyed duplex stainless steel andthus has the worst hot workability among the duplex stainless steelsmentioned previously. Therefore, it is of limited utility for castingproducts. In addition, at the time of manufacturing products by casting,if cooling rate is slow (or if the size of a product is large), due tolarge quantities of Mo, formation of sigma phase is promoted, therebydeteriorating mechanical properties and corrosion resistance of thesteel.

[0014] A conventional method for improving hot workability in duplexstainless steels involves adding Ce into the duplex stainless steels (J.L. Komi et al., Proc. of Int'l Conf. on Stainless Steel, ISIJ Tokyo, p807, (1991) or U.S. Pat. No. 4,765,953). According to this method, the Scontent is lowered to 30 ppm, and Ce is added, so that the segregationof S is prevented, thereby improving the hot workability. However, inthe case where hot workability is improved by adding rare earth elementssuch as Ce in large quantities, use of expensive Ce is unfavorable froman economic point of view. In addition to the above, the use of Ce has aproblem in that strong oxidizing power of Ce causes clogging of nozzlesupon continuous casting. As a result, the manufacture of billet or slabbecomes hard. This duplex stainless steel does not contain W, but Mo.

DISCLOSURE OF THE INVENTION

[0015] Therefore, the present invention has been made in view of theabove problems, and it is an object of the present invention to providea duplex stainless steel with excellent strength, corrosion resistance,and castability, in particular, excellent hot workability, and a methodfor manufacturing the same.

[0016] In accordance with one aspect of the present invention, the aboveand other objects can be accomplished by the provision of a duplexstainless steel comprising (in weight %): less than 0.1% of C; 0.05-2.2%of Si; 2.1-7.8% of Mn; 20-29% of Cr; 3.0-9.5% of Ni; 0.08-0.5% of N;less than 5.0% of Mo and 1.2-8% of W, alone or composite; the balance Feand inevitable impurities. The duplex stainless steel of the presentinvention is grouped into 4 classifications, according to the additiontype of Mo and W.

[0017] First is a low chromium, Mo-containing duplex stainless steel,and comprising (in weight %): less than 0.1% of C; 0.05-2.2% of Si;2.1-7.8% of Mn; 20-26% of Cr (except 26%); 4.1-8.8% of Ni; 0.08-0.345%of N; less than 5.0% of Mo; the balance Fe and inevitable impurities.

[0018] Second is a high chromium, Mo-containing duplex stainless steel,and comprising (in weight %): less than 0.1% of C; 0.05-2.2% of Si;3.1-7.8% of Mn; 26-29% of Cr; 4.1-9.5% of Ni; 0.08-0.345% of N; lessthan 5.0% of Mo; the balance Fe and inevitable impurities.

[0019] Third is a W-containing duplex stainless steel, and comprising(in weight %): less than 0.1% of C; 0.05-2.2% of Si; 2.1-7.8% of Mn;20-29% of Cr; 3.0-9.5% of Ni; 0.08-0.5% of N; 1.2-8% of W; the balanceFe and inevitable impurities.

[0020] Fourth is a Mo—W containing duplex stainless steel, andcomprising (in weight %): less than 0.1% of C; 0.05-2.2% of Si; 2.1-7.8%of Mn; 20-27.8% of Cr; 3.0-9.5% of Ni; 0.08-0.5% of N; less than 0.5% ofMo; 1.2-8% of W; the balance Fe and inevitable impurities, the Mo and Wcontents meeting the conditions: Mo+0.5W 0.8-4.4%.

[0021] In accordance with another aspect of the present invention, thereis provided a method for manufacturing the duplex stainless steel,comprising solution heating the duplex stainless steel compositionmentioned above at, a temperature of 1,050 to 1,250° C.

[0022] In accordance with yet another aspect of the present invention,there is provided a method for manufacturing the duplex stainless steel,comprising the steps of: solution heating the duplex stainless steelcomposition mentioned above at a temperature of 1,050 to 1,250° C., hotworking, which is initiated at a temperature of 1,130 to 1,280° C. andthen terminated at a temperature of more than 1,000° C., and thencooling within the temperature range from 1,000 to 700° C. at a coolingrate of more than 3° C./min.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

[0024]FIG. 1 is a graph showing hot workability (reduction in area)according to the content of Mn;

[0025]FIG. 2(a) is a graph showing hot workability (reduction in area)according to the content of Mo, in a low Mn-containing duplex stainlesssteel and a high Mn-containing duplex stainless steel;

[0026]FIG. 2(b) is a graph showing hot workability (reduction in area)according to the content of Mn, in the case where the content of Mo isconstant;

[0027]FIG. 3 is a graph showing hot workability (reduction in area)according to the content of W, in a low Mn-containing duplex stainlesssteel and a high Mn-containing duplex stainless steel;

[0028]FIG. 4 is a graph showing hot workability (reduction in area)according to temperature, in the inventive steel and comparative steel;

[0029]FIG. 5(a) is a photograph showing interior of the cast slabs ofconventional steel; and

[0030]FIG. 5(b) is a photograph showing interior of the cast slabs ofthe inventive steel.

BEST MODE FOR CARRYING OUT THE INVENTION

[0031] Hereinafter, the present invention will be described in detail.

[0032] The present inventors have discovered that if the content of Cuis limited to 0-1.0% and the content of Mn is increased, hot workabilityis improved. Based on this fact, they have found approaches forimproving hot workability in Mn—Mo, Mn—W and Mn—Mo—W type duplexstainless steels and as a result, completed the present invention.

[0033] (1) Relation Between Mn and Hot Workability in Duplex StainlessSteel

[0034] U.S. Pat. No. 4,657,606 insured room-temperature ductility byadding 5-7% of Mn in a duplex stainless steel of (23-27 wt %)Cr-(4-7 wt%)Ni-(2-4 wt %)Mo-(1.1-3 wt %)Cu. However, there was no mention of howMn influences hot workability (hot ductility). Generally, it is knownthat Mn adversely affects hot workability in duplex stainless steels.

[0035] Generally, room-temperature ductility and hot ductility areindicators of ductility and are similar with respect to test type.However, as shown in Table 1, % reduction in area is a measure of hotductility, while % elongation is a measure of room-temperatureductility, and thus they are different in value. TABLE 1Room-temperature Hot ductility ductility (reduction in Steel (elongation%) area, %: 1,050° C.) Fe-(21-23 wt %)Cr- 30% 41% (4.5-6.5 wt %)Ni-(2.5-3.5 wt %)Mo- (0.08-0.20 wt %)N (SAF2205) Pe-25 wt % Cr-7 wt %  6%58% Ni-4 wt % Mo-1 wt % W-0.3 wt % N-1.5 wt % Si-1.5 wt % Mn

[0036] In an attempt to improve hot workability of duplex stainlesssteels, the present inventors have discovered that Mn adversely affectshot workability in a high Mn-containing duplex stainless steel, in whichmore than 1.1% of Cu is added, while if the content of Cu is lowered to0-1.0%, Mn enhances hot workability. Further, they focused on the factthat Mo and W affect properties of Mn.

[0037] (2) Hot Workability in a Mo-Containing (no W) Duplex StainlessSteel

[0038] As shown in FIG. 1, as the added amount of Mn increases, hotworkability (reduction in area) also increases, regardless of addedamount of alloy and concentration of nitrogen. A-type, which is low inadded amount of alloy and concentration of nitrogen, undergoes a greaterreduction in area than B-type.

[0039]FIG. 2(a) is a graph showing hot workability (reduction in area)according to added amount of Mo, in a low Mn-containing duplex stainlesssteel and a high Mn-containing duplex stainless steel. As the addedamount of Mo decreases, hot workability is improved.

[0040] That is, in a Mo-containing duplex stainless steel, as thecontent of Mn increases, hot workability is improved, in the case wherethe content of Mo is constant. While, in the case where the content ofMn is constant, as the content of Mo increases, hot workability becomesworse. Accordingly, hot workability can be more stably obtained byadjusting the two-component balance of Mn and Mo in a Mo-containingduplex stainless steel. In accordance with the present invention, inorder to secure a reduction in area greater than 50% at 1,050° C., theduplex stainless steel should meet the following formula:

RA(%)=44.37+9.806[% Mn]−3.08[% Mo]−0.76[% Mn][% Mo]≧50

[0041] (3) Hot Workability in a W-Containing Duplex Stainless Steel

[0042] As shown in Table 3, in a high Mn-containing duplex stainlesssteel, as the content of W increases, hot workability (reduction inarea) is improved, while, in a low Mn-containing duplex stainless steel,as the content of W increases, hot workability is lowered. That is, in ahigh Mn-containing duplex stainless steel, W and Mn have a synergisticeffect on improvement of hot workability. The synergistic effect of Wand Mn is also applied to a Mo—W containing duplex stainless steel inthe same manner.

[0043] The present invention has been completed based on the results of(1), (2) and (3) above. Now, the components and compositions of theduplex stainless steel according to the present invention will bedescribed in detail.

[0044] Carbon (C): Less than 0.1%

[0045] C is a strong carbide former, which binds with carbide formingelements such as Cr, Mo, W, Nb and V, contributing to high hardness ofmaterials. However, if carbon is excessively added, it is precipitatedin the form of excess carbide at the ferrite-austenite phase boundaries,with the result that corrosion resistance is lowered. In the presentsteel, if carbon is added in amounts more than 0.1%, it is easilyprecipitated in the form of coarse chromium carbide at the grainboundaries. As a result, the content of chromium is lowered around thegrain boundaries, thereby corrosion resistance being lowered. Therefore,it is preferable to limit the content of carbon to less than 0.1%.Furthermore, in order to maximize strength and corrosion resistance, thecontent of carbon should be limited to less than 0.03%.

[0046] Silicon (Si): 0.05 to 2.2%

[0047] Si acts as a deoxidizing agent and improves the fluidity of themolten steel. For this purpose, Si must be added in an amount of atleast 0.05%. However, when the content of Si exceeds 2.2%, mechanicalproperties in relation to impact toughness are drastically reduced.

[0048] Manganese (Mn): 2.1 to 7.8%

[0049] In conventional duplex stainless steels, Mn was considered to beharmful in hot workability. Therefore, Mn was added in an amount of0.4-1.2%, so as only to adjust deoxidation, desulfurization or thefluidity of the molten metal. In contrast, in the steel of the presentinvention, Mn is positively employed since Mn acts synergistically withMo and W to improve hot workability. Further, Mn can replace expensiveNi, which is desirable from an economic point of view. Generally, it isknown that austenite phase stabilizing ability of Mn is 50% of that ofNi. For these effects, in the steel of the present invention, Mn isadded in an amount of at least 2.1%. However, if the content of Mnexceeds 7.8%, during hot working of slab or billet, the surface of theslab or billet is severely oxidized. Further, oxidation scale thusgenerated lowers production yield, and removal of the oxidation scale isalso difficult. Within the above defined range, Mn improves fluidityupon casting and thus is suitable for casting into thin or intricatelyshaped structures.

[0050] In the Mo-containing (no W) duplex stainless steel of the presentinvention, in the case where the content of Cr is as high as 26-29%, thelower limit of Mn is preferably set to 3.1%, so as to control excessiveincrease of the percentage of ferrite phase.

[0051] Nickel (Ni): 3.0 to 9.5%

[0052] Ni is an austenite stabilizing element. In the steel of thepresent invention, because Mn serves to somewhat stabilize the austenitephase, considering the balance between austenite stabilizers and ferritestabilizers, the content of Ni is preferably limited to 3.0-9.5%. In theMo-containing (no W) duplex stainless steel of the present invention,preferably, where the content of Cr is 20-26% (except 26%), the contentof Ni is set to 4.1-8.8%, while where the content of Cr is 26-29%, thecontent of Ni is set to 4.1-9.5%.

[0053] Chromium (Cr): 20 to 29%

[0054] Cr is a ferrite stabilizing element. It is an essential elementfor improving corrosion resistance and establishing duplex phasestructure consisting of ferrite phase and austenite phase. If thecontent of Cr is less than 20%, the duplex stainless steel cannot havethe required corrosion resistance. On the other hand, if Cr exceeds 29%,the formation of sigma phase is promoted and brittleness increases.Also, low-temperature brittleness occurs around 475° C.

[0055] Nitrogen (N): 0.08 to 0.5%

[0056] N is a strong austenite stabilizing element and reduces the useof expensive Ni, similar to Mn. Also, N is effective for improving thepitting corrosion resistance and corrosion resistance. Generally, 0.02%of N is added to stainless steel materials as impurity. For the abovepurposes, however, N should be added in an amount of at least 0.08%.However, if the content of N exceeds 0.5%, corrosion resistanceincreases but casting defects such as blow holes and like are likely tobe present during ingot casting or continuous casting, thereby degradingquality of steel. Meanwhile, in the Mo-containing (no W) duplexstainless steel of the present invention, if the content of N exceeds0.345%, hot workability is deteriorated.

[0057] To the components defined above, Mo and W are added, alone or incombination.

[0058] Molybdenum (Mo): Less than 5.0%

[0059] Mo is a ferrite stabilizing element and corrosion resistanceimproving element. In particular, Mo improves critical corrosionresistance at certain acidities. However, if the content of Mo exceeds5.0%, formation of sigma phase is likely to result during casting or hotworking, thereby strength and toughness being drastically lowered. Ifhigher corrosion resistance is required, the content of Mo is preferablyset to more than 1.0%.

[0060] In the Mo-containing (no W) duplex stainless steel of the presentinvention, the two-component balance of Mn and Mo should be consideredin order to more stably secure hot workability. In order to secure areduction in area greater than 50% at 1,050° C., the duplex stainlesssteel should meet the following formula, which is obtained from thegraph of FIG. 2:

RA(%)=44.37+9.806[% Mn]−3.08[% Mo]−0.76[% Mn][% Mo]≧50

[0061] Tungsten (W): 1.2 to 8%

[0062] W is a ferrite stabilizing element and corrosion resistanceimproving element. In particular, W improves critical corrosionresistance at certain acidities. Also, W enhances hot workability in ahigh Mn-containing duplex stainless steel. However, if the content of Wis less than 1.2%, the above mentioned effects become insufficient,while if the content of W exceeds 8%, formation of sigma phase is likelyto result during casting or hot working, thereby strength and toughnessbeing drastically lowered. The reason why the upper limit of W is higherthan that of Mo, is that the heavy atomic weight of W makes it difficultto diffuse, thereby delaying the formation of sigma phase in such higherW content. And, in the case where W is added in the same weight ratio asMo, atomic ratio of W to Mo corresponds to about 1 to 2, thereby givingthe same effect as cutting the amount of W added by half. Therefore, thebalance percentage of ferrite phase and austenite phase is little ofconcern here. Considering the above aspect, when Mo and W arecompositely added, their contents should meet the following relation:Mo+0.5W=0.8-4.4%, so as to secure more corrosion resistance.

[0063] P, S and O are added to the duplex stainless steel of the presentinvention as impurities. Their contents should be preferably minimized.

[0064] Phosphorus (P): Less than 0.03%

[0065] Because P is segregated in the grain boundaries or phaseboundaries and thus corrosion susceptibility increases and toughnessdeteriorates, it must be added in as small amounts as possible. However,if the content of P is too low, refining cost becomes too high.Therefore, it is preferable to limit P to less than 0.03%.

[0066] Sulfur (S): Less than 0.03%

[0067] S deteriorates hot workability or forms MnS, thereby decreasingcorrosion resistance. Thus, it is preferable to define the content of Sas low as possible, i.e., less than 0.03%. In particular, in order toobtain higher corrosion resistance, it is preferable to limit S to lessthan 0.003%.

[0068] Oxygen (0): Less than 0.025%

[0069] O forms an oxide type non-metallic inclusion, deterioratingpurity of the steel. Because 0 adversely influences bendability andpress castability, it is preferable to define the content of 0.0 as lowas possible. Therefore, the upper limit of 0 is 0.025%.

[0070] In the duplex stainless steel of the present invention, thecorrosion resistance is greatly affected by the elements Cr, Mo, W andN. Corrosion resistance is described as PREN (Pitting ResistanceEquivalent Number). If PREN is more than 35, the steel is considered tohave a high corrosion resistance, while if it is less than 35, the steelis considered to have a low corrosion resistance.

PREN=% Cr+3.3(% Mo+0.5% W)+30% N

[0071] In order to improve the corrosion resistance and hot workabilityof the present steel with above composition better, the alloy elementssuch as Cu, Ca, B, Mg, Al, Ce, Nb, V, Zr, Ti and Ta can be furtheradded.

[0072] Copper (Cu): Less than 1.0%

[0073] Cu is an austenite stabilizing element. Cu forms a protectivelayer, improving corrosion resistance, and is precipitated in the formof Cu complex particle, increasing strength. However, if the content ofCu exceeds 1.0%, hot workability is markedly deteriorated.

[0074] One Element or More than Two Elements Selected from the GroupConsisting of Nb, V, Zr, Ti and Ta

[0075] Nb, V and Zr form Nb(CN), V₄(CN)₃ and Zr(CN) carbides,respectively. They can be added to control formation of Cr type carbide(M₂₃C₆), thereby preventing formation of corrosion in the grainboundaries. In addition to the above effects, they increase strength bysolution strengthening and particle reinforcement. However, if thecontent of each of Nb and V exceeds 0.4% or if the content of Zr exceeds1.0%, the above carbides are formed coarsely, causing the reduction oftoughness and ductility. Ti and Ta are added in order to controlcorrosion susceptibility in the grain boundaries or reinforce strengtheffectively. For this purpose, each of Ti and Ta should be added in anamount of less than 0.4%.

[0076] One Element or More than Two Elements Selected from the GroupConsisting of Ca, B, Mg, Al and Ce.

[0077] When each of Ca, B and Mg is added to be 0.001-0.01%, or Ce isadded to be less than 0.18%, excellent hot workability can be obtained.If the content of each of Ca, B and Mg is less than 0.001%, the additioneffect is insignificant, while if it exceeds 0.01%, injection into themolten steel is very difficult and no additional effect is seen. Inparticular, Ca and B form coarse oxide inclusions or borides, therebydeteriorating hot workability. If the content of Ce exceeds 0.18%,coarse oxides are widespread, thereby deteriorating hot workability. IfAl is added in an amount of 0.001-0.05%, deoxidation is promoted,thereby more purified casting products being obtained and hotworkability being improved. However, if the content of Al exceeds 0.05%,in a high nitrogen-containing duplex stainless steel such as the steelof the present invention, AIN is formed, thereby deterioratingtoughness. Also, the content of solid-soluble nitrogen is reduced andthus, corrosion resistance is reduced.

[0078] The steel with the above mentioned composition can bemanufactured into casting products by casting, or into finished productforms such as plate, wire, bar and pipe by hot working such as forging,rolling and extrusion. Also, the present steel can be used as a material(wire) for hardfacing, which is suitable for enhancing physicalproperties of the surface of common carbon steel.

[0079] Upon manufacturing the steel into the casting products orfinished product forms, in order to remove sigma phase, segregation ordeformation texture, solution heat treatment can be done at atemperature of 1,050 to 1,250° C. If the temperature is less than 1,050°C., the sigma phase is easily formed and thus the corrosion resistancedeteriorates. On the other hand, if the temperature exceeds 1,250° C.,the percentage of austenite phase increases excessively, therebystrength decreasing and heat treatment cost increasing tremendously.Also, the solution heat treatment makes it possible to remove texturesadversely affecting corrosion resistance of duplex stainless steel andthus increase corrosion resistance still more.

[0080] In particular, in the case where the steels are manufactured intothe finished product forms (plate, wire, and bar), the solution heattreatment is followed by hot working. Preferably, hot working isinitiated at a temperature of 1,130 to 1,280° C. and is terminated at atemperature of more than 1,000° C. As can be seen from FIG. 4, reductionin area is highest at a temperature of 1,130 to 1,280° C., andtermination temperature of hot working is preferably more than 1,000° C.Cooling after the hot working is preferably carried out within thetemperature range from 1,000 to 700° C. at a cooling rate of more than3° C./min. If the cooling rate is less than 3° C./min within the abovementioned temperature range, precipitates, including mainly sigma phase,increase.

[0081] The following examples are given only as an illustration of thepresent invention and are not intended to be construed as a limitationthereof.

EXAMPLE 1

[0082] Various steels, each having the composition as shown in Table 2below, were melted and cast into ingots in a vacuum furnace. The ingotswere then solution heated at a temperature of 1,150° C. in a heatingfurnace for 2 hours to obtain specimens. In carrying outroom-temperature tensile test, the ingots or specimens were solutionheated under the conditions mentioned previously and then water cooled.Corrosion resistance was measured as weight loss at room temperature in10% FeCl₃-6H₂O solution for 72 hours. Corrosion rates of each of thetested steels are summarized in Table 3, below. TABLE 2 Chemicalcomposition (wt %) Steel C Si Mn Cr W Mo Ni N Cu V Nb Ti Ta Inventive 10.027 0.8 4.2 22.5 5.0 — 4.3 0.22 — Inventive 2 0.030 0.8 4.6 21.3 4.50.55 4.3 0.23 0.45 Inventive 3 0.029 0.9 4.8 23.5 4.8 0.58 4.5 0.20 0.48Inventive 4 0.032 0.8 4.6 27.1 3.5 0.46 4.8 0.20 0.51 Inventive 5 0.0280.8 4.7 24.9 4.7 0.45 4.4 0.14 0.50 Inventive 6 0.035 0.8 4.6 25.4 4.60.49 4.3 0.18 0.46 Inventive 7 0.031 0.8 4.5 24.8 4.6 0.57 4.4 0.22 0.49Inventive 8 0.030 0.8 4.5 25.1 2.0 0.44 3.9 0.21 0.48 Inventive 9 0.0320.8 5.0 21.9 6.1 0.45 4.3 0.23 0.47 0.1 Inventive 10 0.033 0.8 4.6 26.54.5 0.46 4.7 0.21 0.48 0.1 0.1 0.05 — Comparative 1 0.028 0.6 0.8 17.2 —2.50 12.2 0.02 Comparative 2 0.075 0.6 0.8 17.1 — 2.45 12.1 0.02

[0083] TABLE 3 Yield strength Corrosion rate Steel (Mpa) Elongation (%)(mm/year) Inventive 1 560 32.0 0.196 Inventive 2 575 30.1 0.228Inventive 3 596 29.7 0.206 Inventive 4 580 29.2 0.105 Inventive 5 70012.6 0.212 Inventive 6 678 13.4 0.124 Inventive 7 649 19.0 0.082Inventive 8 605 32.0 0.244 Inventive 9 635 26.4 0.089 Comparative 1 22055.0 0.617 Comparative 2 290 52.0 0.702

[0084] As can be seen from the Table 3, austenite stainless steels(comparative 1 and 2), which are most widely used in industrial fields,had yield strengths of about 220-290 MPa and room-temperature ductilityof more than 50%. In contrast, the inventive steels had yield strengthsof 575-700 MPa, which is more than 2 times that of comparative steels,and excellent room-temperature ductility of 12-32%.

[0085] As a result of measurement of weight loss by corrosion in 10%FeCl₃.6H₂O solution, comparative steels were all severely corroded, at0.617-0.702 mm/year. However, corrosion rates of the inventive steelswere 0.082-0.244 mm/year. That is, the corrosion resistances of theinventive steels are 3 to 9 times better than comparative steels. Fromthe above results, it can be seen that the inventive steels have bothincreased strength and enhanced corrosion resistance.

EXAMPLE 2

[0086] Inventive steels from Table 2 were solution heated under theconditions of Table 4 below, and then their mechanical properties andcorrosion rates were measured. The results are presented in Table 4below. TABLE 4 Heat Corrosion treatment Yield strength Elongation rateSteel condition (MPa) (%) (mm/year) Comparative As cast state 606 14.80.285 Comparative 950° C./2 hr 641 13.2 0.325 Inventive 1,150° C./2 hr659 20.2 0.067 Inventive 1,250° C./2 hr 649 19.0 0.082

[0087] As shown in Table 4, the inventive steels which had been solutionheated, had higher room-temperature ductility as well as superiorcorrosion resistance, than comparative steels in an as-cast state.

[0088] Consequently, the inventive steels have equal or superiorcorrosion resistance relative to conventional steels, such as 304 or 316type austenite stainless steels, and are excellent in strength.Therefore, the inventive steels can extend lifetimes of chemicalequipments, electric power stations, and marine related equipments, andcontribute to enhancement of working efficiency.

EXAMPLE 3

[0089] Various duplex stainless steels, each having the composition asshown in Table 5 below, were melted and cast into ingots in a vacuumfurnace. The ingots were then solution heated at a temperature of 1,150°C. in a heating furnace for 2 hours to obtain specimens. In carrying outroom-temperature tensile test, the ingots or specimens were, solutionheated under the conditions mentioned previously and then water cooled.Corrosion resistance was measured as weight loss at room temperature in10% FeCl₃.6H₂O solution for 72-hours. Corrosion rates of each of thetested steels are summarized in Table 6, below. The inventive steelsfrom Table 5 all are high corrosion resistant duplex stainless steels,which have PREN values of more than 35. TABLE 5 Chemical composition (wt%) Steel C Si Mn Cr W Mo Ni N Cu V Nb Ti Ta Invetion 1 0.030 0.81 3.7825.22 5.10 — 5.01 0.30 0.5 Invetion 2 0.018 0.80 4.08 24.97 4.35 0.454.69 0.27 0.5 Invetion 3 0.032 0.82 4.64 24.96 4.50 0.48 4.57 0.27 0.5Invetion 4 0.049 0.81 4.80 24.80 4.52 0.56 4.40 0.27 0.5 Invetion 50.092 0.80 4.61 24.96 4.64 0.48 4.37 0.29 0.5 Invetion 6 0.032 0.86 4.8023.45 4.81 0.58 4.52 0.30 0.5 Invetion 7 0.032 0.78 4.60 27.08 4.61 0.464.50 0.32 0.5 Invetion 8 0.033 0.77 4.50 29.10 4.56 0.44 4.40 0.32 0.5Invetion 9 0.035 0.81 4.50 24.90 4.51 0.44 4.42 0.36 0.5 Invetion 100.036 0.81 4.49 24.95 4.62 0.45 4.43 0.45 0.5 Invetion 11 0.032 0.804.48 24.97 6.09 0.45 4.33 0.30 0.5 0.1 Invetion 12 0.031 0.78 4.58 25.024.39 0.46 4.38 0.32 0.5 0.1 0.1 0.05 Comparative 1 0.028 0.60 0.80 17.20— 2.50 12.2 0.02 Comparative 2 0.075 0.60 0.80 17.10 — 2.45 12.1 0.02Comparative 3 0.030 0.79 4.63 25.43 4.60 0.49 4.35 0.18 Comparative 40.031 0.81 4.45 24.55 4.52 0.37 4.40 0.22 Comparative 5 0.030 0.80 4.5025.14 2.03 0.44 4.46 0.26 Comparative 6 0.030 0.80 4.62 21.30 4.59 0.554.30 0.24

[0090] TABLE 6 Yield strength Corrosion rate Steel (MPa) Elongation (%)(mm/year) Inventive 1 550 23.0 0.022 Inventive 2 521 21.1 0.037Inventive 3 630 20.0 0.057 Inventive 4 689 17.5 0.052 Inventive 5 65518.0 0.026 Inventive 6 620 30.0 0.005 Inventive 7 690 19.3 0.038Inventive 8 730 18.7 0.028 Inventive 9 620 32.0 0.043 Inventive 10 55534.5 0.013 Inventive 11 663 24.4 0.021 Inventive 12 657 25.4 0.031Comparative 1 220 55.0 0.617 Comparative 2 290 52.0 0.702 Comparative 3680 8.6 0.195 Comparative 4 649 18.9 0.121 Comparative 5 600 27.2 0.198Comparative 6 565 29.5 0.205

[0091] As can be seen from Table 6, austenite stainless steels(comparative 1 and 2), which are most widely used in industrial fields,had yield strengths of about 220-290 MPa, and room-temperature ductilityof more than 50%. In contrast, the inventive steels had yield strengthsof 520-730 MPa, which is 2 times higher than that of comparative steels,and excellent room-temperature ductility of 17.5-34.5%.

[0092] As a result of measurement of weight loss by corrosion in 10%FeCl₃.6H₂O solution, comparative steels 1 and 2 were severely corroded,at. 0.617-0.702 mm/year. However, corrosion rate of the inventive steelswas 0.005-0.057 mm/year. That is, the corrosion resistances of theinventive steels are 10 to 100 times that of comparative steels. Fromthe above results, it can be seen that the inventive steels have bothincreased strength and enhanced corrosion resistance.

[0093] Comparative steels 3 and 4, which are lower than the inventivesteels in nitrogen content, had poor corrosion rates of 0.121-0.195mm/year. That is, the corrosion resistances of the comparative steels 3and 4 are ⅓ to {fraction (1/24)} that of the inventive steels.Comparative steels 5 and 6, in which the content of W or Cr is low, hadcorrosion resistances only ¼ to {fraction (1/40)} that of the inventivesteels. Although comparative steels 3 to 6 are equal to the inventivesteels with respect to yield strength and elongation, due to their lowcorrosion resistance, they cannot be applied to structural partsrequiring high corrosion resistance.

[0094] Consequently, the inventive steels have superior corrosionresistance relative to conventional steels, such as 304 or 316 typeaustenite stainless steels, or SAF 2205, and are excellent in yieldstrength. Therefore, the inventive steels can extend lifetimes ofchemical equipments, electric power stations, and marine relatedequipments, and contribute to enhancement of working efficiency.

EXAMPLE 4

[0095] Various duplex stainless steels and three kinds of commercialaustenite stainless steels, each having the composition as shown inTable 7 below, were melted and cast into ingots in a vacuum furnace. Theingots were then solution heated at a temperature of 1,100-1,200° C. ina heating furnace for 2 hours to obtain specimens.

[0096] In carrying out room-temperature tensile test, the ingots orspecimens were solution heated under the conditions mentioned previouslyand then water cooled. Corrosion resistance was measured as specimen'sweight loss at room temperature in 10% FeCl₃.6H₂O solution for 72 hours.Corrosion rates of each of the tested steels are summarized in Table 7,below. Meanwhile, the specimens were manufactured into 10 mm indiameter×120 mm in length tensile specimens in the form of bar, and thenwere hot tensile tested by local heating at 1050° C. Then, hotworkability was investigated by measuring a reduction in area. Thereason why the hot workability is investigated using specimens obtainedfrom solution heat treatment of ingots, is that hot working processesare conventionally performed immediately after casting into ingots andthen solution heating of the ingots. The yield strength and hotworkability of the inventive steels are remarkably enhanced after hotworking, compared with solution heated steels. This is because if steelis subjected to hot working process, its internal texture becomes evenfiner. Separately, room-temperature tensile test was conducted using aplate type tensile specimen of more than 25 mm in gauge length×crosssection of 3 mm in thickness×5 mm in width. TABLE 7 Hot Corrosion YieldChemical composition (wt. %) work, rate Strength Steel C Si Mn Cr W MoNi Cu N Others (%) (mm/year) (MPa) Speci.1 0.022 0.4 0.77 23.1 — 3.275.53 — 0.15 — 41 0.352 545 X Speci.2 0.022 0.4 0.79 23.0 — 3.15 8.40 —0.15 — 27 — 410 X Speci.3 0.031 0.8 0.98 25.2 — 4.10 6.86 — 0.26 — 380.016 605 X Speci.4 0.035 0.8 1.00 25.7 — 3.20 5.60 1.80 0.20 — 46 0.032680 X Speci.5 0.035 0.8 0.99 21.9 — 5.01 7.18 — 0.24 — 35 0.022 545 XSpeci.6 0.027 0.6 4.15 23.0 — 3.12 5.45 — 0.15 — 66 0.315 550 ◯ Speci.70.025 0.6 4.52 22.9 — 3.10 8.47 — 0.15 — 58 — 415 ◯ Speci.8 0.023 0.52.41 23.0 — 3.02 8.72 — 0.16 0.0035Ca 57 — 408 ◯ 0.0042B Speci.9 0.0220.5 2.53 22.9 — 3.05 8.60 — 0.16 0.0035Mg 57 — 495 ◯ 0.0034B Speci.100.025 0.5 2.63 23.0 — 3.12 8.68 — 0.16 0.0022Mg 67 — 488 ◯ Speci.110.022 0.4 3.52 23.0 — 3.10 8.63 — 0.16 0.0043B 55 — 445 ◯ Speci.12 0.0260.6 3.05 25.2 — 4.15 7.05 — 0.30 — 54 — 540 ◯ Speci.13 0.062 0.8 0.9424.4 5.21 — 6.19 0.46 0.29 — 35 0.023 560 X Speci.14 0.028 0.8 4.52 24.26.02 — 4.75 — 0.26 — 66 0.022 612 ◯ Speci.15 0.022 0.4 0.80 22.7 2.511.49 5.54 — 0.16 — 49 — 490 X Speci.16 0.023 0.4 0.81 22.7 2.55 1.488.88 — 0.15 — 37 — 410 X Speci.17 0.032 0.8 0.94 24.4 3.51 0.76 7.190.46 0.29 — 35 0.023 545 X Speci.18 0.032 0.8 0.98 24.6 3.30 2.67 6.901.33 0.29 — 21 0.015 640 X Speci.19 0.032 0.8 0.96 24.9 2.09 3.09 7.100.45 0.27 — 45 0.021 642 X Speci.20 0.018 0.8 4.08 25.0 4.35 0.45 4.690.48 0.27 — 65 0.118 521 ◯ Speci.21 0.032 0.8 4.64 25.0 4.30 0.48 4.570.49 0.27 — 61 0.177 630 ◯ Speci.22 0.049 0.8 4.80 24.8 4.52 0.56 4.400.48 0.27 — 55 0.082 689 ◯ Speci.23 0.092 0.8 4.61 25.0 4.64 0.48 4.370.49 0.29 — 58 0.036 655 ◯ Speci.24 0.030 0.8 4.62 21.3 3.59 0.55 4.300.49 0.24 — 55 0.077 575 ◯ Speci.25 0.032 0.9 4.80 23.5 4.81 0.58 4.520.49 0.30 — 54 0.007 596 ◯ Speci.26 0.032 0.8 4.60 27.1 4.61 0.46 4.500.48 0.32 — 63 0.009 580 ◯ Speci.27 0.030 0.8 4.45 24.9 4.62 0.49 4.400.50 0.18 — 78 0.346 678 ◯ Speci.28 0.031 0.8 4.63 25.4 4.60 0.57 4.350.49 0.22 — 67 0.082 649 ◯ Speci.29 0.022 0.6 3.10 23.5 4.52 0.72 4.510.48 0.21 — 63 0.092 632 ◯ Speci.30 0.025 0.7 2.31 23.5 5.01 0.65 4.520.47 0.23 — 58 0.095 650 ◯ Speci.31 0.035 0.8 4.50 24.9 4.51 0.44 4.420.47 0.36 — 52 0.043 620 ◯ Speci.32 0.036 0.8 4.49 25.0 4.62 0.45 4.430.47 0.45 — 50 0.017 555 ◯ Speci.33 0.030 0.8 4.50 25.1 2.03 0.44 4.460.47 0.26 — 57 0.363 605 ◯ Speci.34 0.032 0.8 4.48 25.0 6.09 0.45 4.330.45 0.30 — 68 0.006 635 ◯ Speci.35 0.030 0.6 4.46 23.2 4.30 0.47 4.290.49 0.34 0.0021Mg 55 — 560 ◯ 0.0034B Speci.36 0.030 0.8 2.51 25.0 3.600.83 7.03 0.52 0.23 0.67Zr 62 0.020 610 ◯ Speci.37 0.043 0.5 2.37 24.03.70 0.80 6.63 0.47 0.31 0.12V 61 0.018 530 ◯ Speci.38 0.031 0.8 2.4925.2 3.52 0.80 6.95 0.51 0.30 0.l3Nb 60 0.022 600 ◯ Speci.39 0.029 0.82.54 25.1 3.41 0.79 7.01 0.51 0.17 0.29Ti 76 0.019 630 ◯ Speci.40 0.0280.7 4.51 24.6 4.52 0.45 4.52 — 0.23 0.05Ta 69 — 657 ◯ Speci.41 0.027 0.84.35 24.3 4.61 0.49 4.57 — 0.23 0.01Ce, 70 — 645 ◯ 0.005A1 316L 0.0280.6 0.80 17.2 — 2.50 12.2 — 0.043 — — 0.617 220 X 316 0.075 0.6 0.8017.1 — 2.45 12.1 — 0.020 — — 0.702 290 X 304 0.030 0.8 1.00 19.3 — 10.7— 0.033 68 7.065 289 X Conven.1 0.030 0.8 5.25 25.2 — 2.51 6.15 2.810.28 28 0.105 455 X Conven.2 0.028 0.8 0.99 25.0 — 4.08 6.99 — 0.31 340.016 610 X

[0097] In Table 7, 316L, 316 and 304 steels are austenite type stainlesssteels, which are most widely used in industrial fields, and have yieldstrengths of about 220-290 MPa. In contrast, the inventive steels are120-400 Mpa higher than these austenite type stainless steels, withrespect to yield strength. The corrosion rates of 316L, 316 and 304steels range from 0.617 to 7.065 mm/year. In contrast, the corrosionrates of the inventive steels range from 0.007 to 0.363 mm/year, showingexcellent corrosion resistance.

[0098] Specimens 1-5 are conventional commercial Mo-containing (no W)duplex stainless steels, and exhibit almost the same yield strength andcorrosion resistance as the inventive steels. In spite of theseadvantages, they have severe problems in that hot workability is verylow and thus defective proportion is very high, in particular in Gingermill. The hot workability (reduction in area) of specimens 1-5 rangesfrom 27 to 46%, very poor values. However, inventive steels with thecontent of: Mn according to the present invention had hot workability(reduction in area) of 52-66%, resulting in enhancement of hotworkability by more than 50%, compared with specimens 1-5.

[0099] Similar results to above were also obtained in W-containing (noMo) duplex stainless steels. Specimen 13 is a W-containing (no Mo)duplex stainless steel. Due to low Mn content, it exhibited very low hotworkability, i.e. about 35%. Specimen 14, of which Mn content is 4.52 wt%, had reduction in area of 66%, which is an enhancement of reduction inarea by 88%, compared with specimen 13.

[0100] Similar results to the above were also obtained from Mo—Wcontaining duplex stainless steels. Specimens 15-19 are conventionalcommercial steels, and their hot workability is very poor, i.e. 21-49%.However, the inventive counterparts, which have Mn contents according tothe present invention, were enhanced by 50-78% with respect to reductionin area. Specifically, specimen 15, which is relatively low in alloyaddition amount and N content, had 49% reduction in area but was thehighest in reduction in area among comparative, low Mn-containing, Mo—Wcontaining duplex stainless steels. Meanwhile, among the inventivecounterparts, specimen 27, which has higher Mn content, had 78%reduction in area, about 59% higher than specimen 15. Specimen 18, whichis relatively high in alloy addition amount and nitrogen content, had21% reduction in area, the worst value. However, specimen 34, which hasa similar composition to specimen 18, had 68% reduction in area,resulting in enhancement of hot workability of more than about 3 times,compared with specimen 18.

[0101]FIG. 1 is a graph showing influence of Mn content on hotworkability in a variety of duplex stainless steels. The inventivesteels exhibit remarkably improved hot workability relative toconventional commercial low Mn-containing stainless steels. In FIG. 1,A-type (specimens 1, 4, 6, 27, etc.) is one alloy group which isrelatively low in alloy addition amount and nitrogen content, and B-type(specimens 5, 17, 12, 34, etc) is another alloy group which isrelatively high in alloy addition amount and nitrogen content. It can beseen from the FIG. 1 that regardless of the alloy addition amount andnitrogen content, as the content of Mn increases, hot workability isgradually improved. This result is utterly opposed to the commonperception that as the content of Mn increases, hot workabilitydecreases.

[0102]FIG. 2(a) is a graph showing influence of Mo on hot workability,in low Mn-containing duplex stainless steels and high Mn-containingduplex stainless steels (specimens 1 to 12). It directly demonstratesthe fact that as the content of Mn increases, hot workability isimproved. As shown in FIG. 2(a), regardless of the content of Mn, as thecontent of Mo increases, hot workability decreases. FIG. 2(b) shows inMo-containing duplex stainless steels that as the content of Mnincreases, hot workability is improved, in the case where the content ofMo is constant.

[0103]FIG. 3 shows hot workability according to the content of W orW—Mo, in W- or W—Mo containing duplex stainless steels (specimens 13 to41). FIG. 3 supports the conclusions of FIG. 1 that as the content of Mnincreases, hot workability is improved. As for conventional 1%Mn-containing steels, as the content of W or W—Mo increases, hotworkability is continuously reduced, while as for inventive highMn-containing steels, as the content of W or W—Mo increases, hotworkability is continuously increased. Accordingly, in inventive steels,in the case where Mn and W are compositely added, hot workability ismore improved even in high alloy addition amount.

[0104] Meanwhile, in Mo-, W-, or W—Mo-containing steels, in the casewhere the content of Cu exceeds 1%, hot workability is very poor, as canbe seen from specimens 4 and 18, and conventional steel 1 (U.S. Pat. No.4,657,606). Consequently, addition of excessive Cu remarkably reduceshot workability.

EXAMPLE 5

[0105] The inventive steel (for example, specimen 28) was cast andsolution heated at a temperature of 1,050 to 1,250° C. Its physicalproperties are presented in Table 8 below.

[0106] As can be seen from the Table 8, strength was excellent, andcorrosion resistance, ductility and impact toughness were improved.TABLE 8 Impact Corrosion Treatment Yield strength Elongation energy ratecondition (MPa) (%) (3) (mm/year) As-cast state 606 14.8 11.6 0.2251,100° C./2 hr 662 19.8 185.0 — 1,150° C./2 hr 659 20.2 — 0.067 1,200°C./2 hr 649 19.0 96.0 0.082

EXAMPLE 6

[0107] The inventive steel (specimen 28) and comparative steel (specimen17) were measured for hot workability. The results are shown in FIG. 4.

[0108] As shown in FIG. 4, it can be seen that the inventive steel issuperior in hot workability to comparative steel. The inventive steel(specimen 28) exhibited reduction in area of 90-99.52%, while thecomparative steel (specimen 17) exhibited reduction in area of 55-83%.Consequently, higher temperatures than in the inventive steel mustinevitably be applied to the comparative steel. That is, in order toadequately hot work the comparative steel, working temperature must beincreased. As a result, there are problems in that excessive energy isconsumed, as well as hot workability is low, resulting in increase ofdefective proportion. The hot working of the inventive steels can beinitiated at lower temperatures.

[0109] Although the hot workability of the inventive steels is superiorto the comparative steels, it reduces below 1000° C. Therefore, hotworking of the inventive steels must be terminated at more than 1000° C.

[0110] Meanwhile, specimen 28 was measured for the amounts ofprecipitates (mainly sigma phase) formed within the temperature rangefrom 1000 to 700° C. at various cooling rates. Then, the specimen 28 wasair cooled from 700° C. to room temperature. The quantitative resultsare shown in Table 9. As shown in Table 9, 6.5% of precipitates areformed at the cooling rate of 1° C./min, 0.8% of precipitates are formedat the cooling rate of 5° C./min, and few precipitates are formed at 50°C./min. In the case where precipitates (mainly sigma phase) are formed,toughness of the steel was drastically deteriorated. As a result,internal cracks were easily formed during cooling and corrosionresistance and cold workability in stainless steel products weredeteriorated. Generally, it is preferred that the amount of theprecipitates is limited to less than 2%. TABLE 9 Cooling rate 1° C./min5° C./min 50° C./min 100° C./min Amount of 6.5 0.8 0 0 precipitates (%)

EXAMPLE 7

[0111] The inventive steel (specimen 29) and conventional steel 2 fromthe Table 7 were cast and internal photographs of the cast slabs areshown in FIG. 5.

[0112] The inventive steel (specimen 29) was excellent in castabilitydue to high Mn content. The inventive steels have an advantage ofreducing occurrence of cracks in the interior of soft billet or ingot,compared with conventional duplex stainless steels. As shown in FIG.5(a), as for conventional steel 2, although hot top sleeves were put ontop of ingot mold in order to avoid formation of shrinkage cavities iningots, shrinkage cavities formed to finally comprise 65% of the wholecast slabs. In contrast, as for the inventive steel (specimen 29, seeFIG. 5(b)), shrinkage cavities formed only 15% of the whole cast slabs.Accordingly, the inventive high Mn-containing steels contribute to thereduction of casting defects.

INDUSTRIAL APPLICABILITY

[0113] As apparent from the above description, the present inventionprovides a duplex stainless steel, which is excellent in corrosionresistance, strength and hot workability, relative to 304 or 316 typeaustenite stainless steels. The duplex stainless steels of the presentinvention are excellent in castability and thus can be easily cast intothin products or intricately shaped products. In particular, due to highhot workability, the duplex stainless steels of the present inventioncan be made into finished product forms, including plate, wire, bar,pipe, and the like.

[0114] Although the preferred embodiments of the present invention havebeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A high manganese duplex stainless steel with excellent hotworkability, comprising (in weight %): less than 0.1% of C; 0.05-2.2% ofSi; 2.1-7.8% of Mn; 20-29% of Cr; 3.0-9.5% of Ni; 0.08-0.5% N; less than5.0% of Mo and 1.2-8% W, alone or composite; the balance Fe andinevitable impurities.
 2. The high manganese duplex stainless steel asset forth in claim 1, which is a low chromium, Mo-containing duplexstainless steel, comprising (in weight %): less than 0.1% of C;0.05-2.2% of Si; 2.1-7.8% of Mn; 20-26% of Cr (except 26%); 4.1-8.8% ofNi; 0.08-0.345% N; less than 5.0% of Mo; the balance Fe and inevitableimpurities.
 3. The high manganese duplex stainless steel as set forth inclaim 1, which is a high chromium, Mo-containing duplex stainless steel,comprising (in weight %): less than 0.1% of C; 0.05-2.2% of Si; 3.1-7.8%of Mn; 26-29% of Cr; 4.1-9.5% of Ni; 0.08-0.345% N; less than 5.0% ofMo; the balance Fe and inevitable impurities.
 4. The high manganeseduplex stainless steel as set forth in claim 1, which is a W-containingduplex stainless steel, comprising (in weight %): less than 0.1% of C;0.05-2.2% of Si; 2.1-7.8% of Mn; 20-29% of Cr; 3.0-9.5% of Ni; 0.08-0.5%N; 1.2-8% of W; the balance Fe and inevitable impurities.
 5. The highmanganese duplex stainless steel as set forth in claim 1, which is aMo—W containing duplex stainless steel, comprising (in weight %): lessthan 0.1% of C; 0.05-2.2% of Si; 2.1-7.8% of Mn; 20-27.8% of Cr;3.0-9.5% of Ni; 0.08-0.5% of N; less than 0.5% of Mo; 1.2-8% of W; thebalance Fe and inevitable impurities, the Mo and W meeting theconditions: Mo+0.5W=0.8-4.4%.
 6. The high manganese duplex stainlesssteel as set forth in claim 1, wherein the content of Mo is 1.0-5.0%. 7.The high manganese duplex stainless steel as set forth in claim 1,wherein the contents of Mo and Mn meet the following formula:44.37+9.806[% Mn]−3.08[% Mo]−0.76[% Mn][% Mo]≧50.
 8. The high manganeseduplex stainless steel as set forth in claim 1, wherein the contents ofCr, Mo, W and N meet the following formula: PREN=% Cr+3.3(% Mo+0.5%W)+30% N≧35.
 9. The high manganese duplex stainless steel as set forthin claim 1, wherein the content of C is less than 0.03%.
 10. The highmanganese duplex stainless steel as set forth in claim 1, which furthercomprises one element or more than two elements selected from the groupconsisting of less than 0.4% of Nb; less than 0.4% of V; less than 1.0%of Zr; less than 0.4% of Ti, and less than 0.4% of Ta.
 11. The highmanganese duplex stainless steel as set forth in claim 1, which furthercomprises less than 1.0% of Cu.
 12. The high manganese duplex stainlesssteel as set forth in claim 1, which further comprises one or twoelements selected from the group consisting of less than 0.18% of Ce;0.001-0.01% of Ca; 0.001-0.01% of B; 0.001-0.01% of Mg; and 0.001-0.05%of Al.
 13. A method for manufacturing a high manganese duplex stainlesssteel, comprising solution heating the duplex stainless steel as setforth in claim 1 at a temperature of 1,050 to 1,250° C.
 14. The methodas set forth in claim 13, which comprises the steps of: solution heatingthe duplex stainless steel as set forth in claim 1 at a temperature of1,050 to 1,250° C., hot working, which is initiated at a temperature of1,130 to 1,280° C. and then terminated at a temperature of more than1,000° C., and then cooling within the temperature range from 1,000 to700° C. at a cooling rate of more than 3° C./min.
 15. The method as setforth in claim 13, wherein the content of Mo is 1.0-5.0%.
 16. The methodas set forth in claim 13, wherein the contents of Mo and Mn meet thefollowing formula: 44.37+9.806[% Mn]−3.08[% Mo]−0.76 [% Mn][% Mo]≧50.17. The method as set forth in claim 13, wherein the contents of Cr, Mo,W and N meet the following formula: PREN=% Cr+3.3(% Mo+0.5% W)+30% N≧35.18. The method as set forth in claim 13, wherein the content of C isless than 0.03%.
 19. The method as set forth in claim 13, wherein thesteel further comprises one element or more than two elements selectedfrom the group consisting of less than 0.4% of Nb; less than 0.4% of V;less than 1.0% of Zr; less than 0.4% of Ti, and less than 0.4% of Ta.20. The method as set forth in claim 13, wherein the steel furthercomprises less than 1.0% of Cu.
 21. The method as set forth in claim 13,wherein the steel further comprises one or two elements selected fromthe group consisting of less than 0.18% of Ce; 0.001-0.01% of Ca;0.001-0.01% of B; 0.001-0.01% of Mg; and 0.001-0.05% of Al.